A radio spectrum needs to be used for wireless communication between two devices. The radio spectrum is roughly classified into a licensed (Licensed) spectrum and an unlicensed (Unlicensed) spectrum.
When wireless communication is performed by using the unlicensed spectrum, a network device needs to perform a clear channel assessment (CCA) on a channel for which the unlicensed spectrum is used. If an assessment result is that the channel is in a non-idle state, it indicates that the channel is being used by another device. If an assessment result is that the channel is in an idle state, the network device first sends a pilot to a terminal device over the channel, and then sends current-frame data to the terminal device over the channel, or receives, over the channel, current-frame data sent by the terminal device. A frame occupies a fixed time period. The time period includes two parts of time. In a first part of time, the network device sends a pilot to the terminal device. In a second part of time, the network device sends data to the terminal device or receives data from the terminal device. An entire process in which the network device performs the CCA is referred to as a listen before talk (LBT) process. A quantity of CCAs in each LBT process is uncertain. Therefore, duration of the LBT process is also uncertain. In addition, duration of each CCA is usually fixed duration, or is possibly unfixed duration.
When the network device is in the LBT process, the terminal device needs to keep listening over the channel, to receive a pilot and data that are sent by the network device. If duration of the LBT process is relatively long and power of the terminal device is supplied by using a battery, a large quantity of electricity of the terminal device is wasted.